Pentafluoroethane is one of the possible replacements for chlorofluorocarbons (CFC), which are the subject-matter of the Montreal protocol and are characterized by an exceptionally long lifetime which allows them to reach the upper layers of the atmosphere and thus play a part, under the influence of UV radiation, in destroying the ozone layer. It is thus obvious that, as a function of the various production processes, their replacements should contain only traces of these CFCs.
The replacements are generally obtained either by appropriate fluorination methods, which are not highly selective and may generate perhalo compounds of the CFC type by dismutation, or are obtained from CFCs themselves by reduction methods, in practice by hydrogenolysis reactions. Thus, pentafluoroethane (F125) may be prepared by fluorination of perchloroethylene or of its intermediate fluorination products such as dichlorotrifluoroethane (F123) and chlorotetrafluoroethane (F124) or by hydrogenolysis of chloropentafluoroethane (F115). In both cases, the F125 produced contains non-negligible amounts of F115 which, since F115 is a CFC, should be removed as fully as possible.
Now, the existence of an F115/F125 azeotrope at 21% by weight of F115 (see U.S. Pat. No. 3,505,233) with a boiling point (-48.5.degree. C. at 1.013 bar) which is very close to that of F125 (-48.1.degree. C.) makes it virtually impossible to separate F115 and F125 completely by distillation. The F115 can thus only be removed from the F125 via a chemical route or by physical methods involving an intermediary substance.
In Patent Application EP 508,631, which describes the production of hydrofluorocarbons (HFC) by liquid-phase chemical reduction of chloro, bromo or iodo compounds with a metal hydride or a complex of such a hydride, it is indicated that this process may be advantageous for purifying certain HFCs such as F125. With the same aim, the Japanese patent application published (Kokai) under the No. 2001414/90 uses metal redox couples in a solvent medium. Other techniques, such as that described in Journal of Fluorine Chemistry, 1991 vol. 55, pp. 105-107, use organic reducing agents such as ammonium formate in DMF medium and in the presence of ammonium persulphate.
These processes, which use reactants that are difficult to handle (metal hydrides) or that are liable to pose effluent problems, are relatively incompatible with industrial production of F125 in large tonnage.
For the industrial manufacture of F125, the technique of extractive distillation appears to be an ideal process for removing the residual F115.
In an extractive distillation process, the constituents of a binary mixture are separated using a so-called extraction column containing successively, from the boiling vessel to the head, three sections, one for depletion, the second for absorption and the third for recovery.
The binary mixture to be fractionated is injected into the top of the depletion section, whereas the intermediary substance acting as selective solvent is introduced into the top of the absorption section so as to circulate in the liquid state from its point of introduction to the boiling vessel.
The third section, known as the recovery section, serves to separate out by distillation the constituent which is least absorbed from the traces of solvent entrained under the effect of its non-zero vapour pressure.
The application of this technique to the purification of 1,1,1,2-tetrafluoroethane (F134a) is the subject of U.S. Pat. No. 5,200,431; the extraction agent used is a chlorinated solvent or an aliphatic hydrocarbon.
The application of extractive distillation to the purification of F125 is already described in U.S. Pat. No. 5,087,329, which uses as extraction agent a C.sub.1 to C.sub.4 fluorohydrocarbon optionally containing hydrogen and/or chlorine atoms and having a boiling point of between -39 and +50.degree. C. According to the data in that patent, the dichlorotetrafluoroethanes (F114 and F114a) are at least three times as effective as the other compounds mentioned. Moreover, 5 of the 8 solvents mentioned are CFCs forming the subject-matter of the Montreal protocol and whose marketing should cease in the near future.
The industrial use of the process according to that patent can therefore be envisaged economically only when the extraction agent used forms part of the chain of intermediates leading to F125, that is to say, in fact, in processes for the preparation of F125 by hydrogenolysis.
In the case of the manufacture of F125 by fluorination of perchloroethylene or of its partial fluorination products (F122, F123, F124), U.S. Pat. No. 5,087,329 only allows a choice between CFCs which will no longer be commercially available and products of lower performance such as F124 or F123.